Digital or solid state lighting technologies, i.e., illumination based on semiconductor light sources, such as light-emitting diodes (LEDs), offer a viable alternative to traditional fluorescent, high-intensity discharge (HID), and incandescent lamps. Functional advantages and benefits of LEDs include high energy conversion and optical efficiency, durability, lower operating costs, and many others. Recent advances in LED technology have provided efficient and robust full-spectrum lighting sources that enable a variety of lighting effects in many applications.
Some of the fixtures embodying these sources feature a lighting module, including one or more LEDs capable of producing white light and/or different colors of light, e.g., red, green and blue, as well as a controller or processor for independently controlling the output of the LEDs in order to generate a variety of colors and color-changing lighting effects, for example, as discussed in detail in U.S. Pat. Nos. 6,016,038 and 6,211,626. LED technology includes line voltage powered luminaires, such as the ESSENTIALWHITE series, available from Philips Color Kinetics. Such luminaires may be dimmable using trailing edge dimmer technology, such as electric low voltage (ELV) type dimmers for 120VAC or 220VAC line voltages (or input mains voltages).
Many lighting applications make use of dimmers. Conventional dimmers work well with incandescent (bulb and halogen) lamps. However, problems occur with other types of electronic lamps, including compact fluorescent lamp (CFL), low voltage halogen lamps using electronic transformers and solid state lighting (SSL) lamps, such as LEDs and OLEDs. Low voltage halogen lamps using electronic transformers, in particular, may be dimmed using special dimmers, such as ELV type dimmers or resistive-capacitive (RC) dimmers, which work adequately with loads that have a power factor correction (PFC) circuit at the input.
Conventional dimmers typically chop a portion of each waveform of the input mains voltage signal and pass the remainder of the waveform to the lighting fixture. A leading edge or forward-phase dimmer chops the leading edge of the voltage signal waveform. A trailing edge or reverse-phase dimmer chops the trailing edges of the voltage signal waveforms. Electronic loads, such as LED drivers, typically operate better with trailing edge dimmers.
Unlike incandescent and other resistive lighting devices which respond naturally without error to a chopped sine wave produced by a phase chopping dimmer, LEDs and other solid state lighting loads may incur a number of problems when placed on such phase chopping dimmers, such as low end drop out, triac misfiring, minimum load issues, high end flicker, and large steps in light output. Some problems involve compatibility among components of the lighting system, such as the phase chopping dimmers and the solid state lighting load drivers (e.g., power converters), and exhibit corresponding symptoms that result in undesirable flicker in the light output. The flicker is typically caused by a lack of uniformity among the chopped sine waves of the rectified input mains voltage signal, where the waveforms are asymmetrical.
For example, FIG. 1A shows waveforms of an unrectified input mains voltage signal input to a phase chopping dimmer, where the unrectified input mains voltage signal has periodically occurring positive and negative half cycles. FIG. 1B shows chopped waveforms of the rectified input mains voltage signal output from the dimmer, where the dimming level is about 50 percent, as indicated by the relative position of the dimmer slider. More particularly, FIG. 1B shows a scenario in which the dimmer and the solid state lighting load driver are functioning correctly, and thus provide substantially uniform rectified chopped sine waves corresponding to the positive and negative half cycles. That is, the dimmed rectified input mains voltage signal has symmetrical chopping of both the positive and negative half cycles of the unrectified input mains voltage.
In contrast, FIG. 1C shows chopped waveforms of the rectified input mains voltage signal output from the dimmer, where the dimmer and the solid state lighting load driver are functioning incorrectly, and thus provide non-uniform rectified chopped sine waves. That is, the dimmed rectified input mains voltage signal has asymmetrical chopping of the positive and negative half cycles of the unrectified input mains voltage. This asymmetrical presentation in the chopped waveforms of the rectified input mains voltage signal results in flickering in the light output at the solid state lighting load.
The improper operation may result from multiple possible problems. One problem is insufficient load current passing through the dimmer's internal switch. The dimmer derives its internal timing signals based on the current going through the solid state lighting load. Because solid state lighting load may be a small fraction of an incandescent load, the current drawn through the dimmer may not be sufficient to ensure correct operation of the internal timing signals. Another problem is that the dimmer may derive its internal power supply, which keeps its internal circuits operating, via the current drawn through the load. When the load is not sufficient, the internal power supply of the dimmer may drop out, causing the asymmetries in the waveforms.
Thus, there is a need in the art to detect improper operation of lighting system components, such as the dimmer and/or the solid state lighting load driver, and to identify and implement corrective action to correct the improper operation and/or remove power to the solid state lighting load, to eliminate undesirable effects, such as light flicker.